The present invention relates generally to a method of fabricating a suspended microstructure. More specifically, the present invention relates to a method of fabricating a suspended microstructure that eliminates the need to form through holes in the suspended microstructure that reduce a useful surface area of the suspended microstructure.
Suspended platforms are useful in applications where the advantages of microelectronic fabrication techniques can be used to form microstructures such as accelerometers, pressure sensors, actuators, fluidic devices, biochemical devices, and miniature machines. Micro-Electro-Mechanical Systems (MEMS) are one example of a system that can incorporate a suspended platform.
For instance, MEMS can integrate micromechanical elements and electronic elements on a common substrate material such as a silicon wafer. Microelectronic fabrication techniques can be used to fabricate the electronic elements such as CMOS circuits, for example. On the other hand, the micromechanical elements can be fabricated using micromachining techniques that deposit layers of materials to form mechanical and electromechanical devices or that selectively etch one or more layers of material such as a layer of silicon or a layer of silicon oxide to form mechanical and electromechanical devices.
A prior method for fabricating a suspended structure from single crystal silicon (Si) is illustrated in FIGS. 1a through 1d. FIG. 1a illustrates a prior structure 100 including an upper wafer 108 having a platform 102 that is suspended by flexures 104. The platform 102 includes several holes 114 that extend all the way through the platform 102. That is, the holes 114 perforate the platform 102.
FIGS. 1b through 1d are a cross-sectional view taken along line AA of FIG. 1a that illustrate a process for fabricating the prior structure 100. The process begins with a bonded silicon-on-insulator wafer 106 that includes the upper wafer 108 that is chemically bonded to a lower wafer 110 by a thin silicon oxide (SiO2) layer 112 (i.e. a layer of dielectric material). The process for forming the bonded silicon-on-insulator wafer 106 are well understood in the microelectronics art.
Next, a top surface 124 of the upper wafer 108 is patterned using conventional photolithography techniques and then the upper wafer 108 is etched to form trenches 103 that define the flexures 104 and the platform 102 as well as a regularly spaced array of the holes 114 as illustrated in FIG. 1c. The trenches 103 extend through the upper wafer 108.
The holes 114 are through holes (i.e. they extend all the way trough the platform 102) and are required to allow the silicon oxide (SiO2) layer 112 to be removed from beneath the platform 108 in a subsequent etching step.
In FIG. 1d, the bonded silicon-on-insulator wafer 106 is exposed to a selective etch material such as hydrofluoric acid (HF). The etch material flows through the holes 114 and dissolves the silicon oxide layer 112 that is beneath the platform 102 thereby freeing the platform 102 from the silicon oxide layer 112. The holes 114 are required in order to reduce the distance an etch front of the etch material must travel to free the platform 102. The silicon oxide layer 112 surrounding the platform 112 is undercut by a distance 122 that is approximately equal to one-half a hole-to-hole spacing 120 (i.e. the space between holes 114, see FIG. 1a).
A major disadvantage to the prior structure 100 is that the holes 114 reduce the surface area available on the platform 102. For instance, in ultrahigh density data storage applications, the platform 102 may include one or more layers of a storage medium that stores data as an alterable state of the storage medium. Optical or electron emission means (i.e. a laser or an electron beam) can be used to read and/or write data to the storage medium. It is undesirable to have the platform 102 perforated with the holes 114 because the holes 114 reduce the surface area of the platform 102 available for the storage medium. Moreover, an addressing scheme for reading or writing data to the storage medium must take into account the locations of the holes 114 to prevent reading or writing to an area in which the storage medium is non-existent. Because the holes 114 only serve to facilitate the removal of the silicon oxide layer 112 from beneath the platform 102, they are a non-functional feature of the platform 102. Accordingly, it is desirable to eliminate the holes 114 as they serve no useful purpose once the platform 102 has been formed.
Another prior method for fabricating a suspended structure from single crystal silicon (Si) is illustrated in FIGS. 2a through 2c. FIG. 2a illustrates a prior structure 200 including an upper wafer 208 having a platform 202 that is suspended by flexures 204.
FIGS. 2b through 2c are a cross-sectional view taken along line AA of FIG. 2a that illustrate a process for fabricating the prior structure 200. The process begins with a bonded silicon-on-insulator wafer 206 that includes the upper wafer 208 that is chemically bonded to a lower wafer 210 by a thin silicon oxide (SiO2) layer 212. In FIG. 2b, prior to bonding the upper wafer 208 to the lower wafer 210, the thin silicon oxide layer 212 is patterned and then etched to form a well. The well becomes a sealed cavity 216 after the upper and lower wafers (206, 208) are bonded to each other.
Next, in FIG. 2c, a top surface 220 of the upper wafer 208 is patterned and then etched to form trenches 203 that define the flexures 204 and the platform 202. The trenches 203 extend through the upper wafer 208 to the sealed cavity 216. As a result of the etching, an upper surface 222 of the lower wafer 210 is exposed to the etching material and is subsequently etched to form shallow pits 218 that extend inward of the upper surface 222 as illustrated in FIG. 2c. 
Consequently, one disadvantage of the method for fabricating the prior structure 200 is that the fabrication results in damage to the upper surface 222 of the lower wafer 210. In some applications the lower wafer 210 may contain buried components such as electrodes, interconnect structures, circuitry, or some other element that is essential to the functioning of the structure 200. Therefore, it is desirable to protect those components during the fabrication process. Conversely, the method for fabricating the prior structure 200 can result in damaging those components because the upper surface 222 is not protected from the etch material during the fabrication process.
Accordingly, there exists a need for a method for fabricating a suspended microstructure that does not require etch holes to remove a layer of material from beneath the suspended microstructure.
There is also a need for a method of fabricating suspended microstructures that protects (i.e. does not damage) an upper surface of a lower wafer from etch materials during the fabrication process so that components that are buried in the lower wafer are not damaged by the etch materials.
A method of fabricating a suspended platform of the present invention solves the aforementioned needs. The method of fabricating a suspended platform according to the present invention does not require etch holes in the suspended platform to remove a layer of material beneath the suspended platform. As a result, the suspended platform is non-perforate and substantially all of the surface area of the suspended platform is available for use.
Moreover, the method of fabricating a suspended platform according to the present invention prevents pitting of an upper surface of a lower wafer by covering the upper surface with a thin layer of material that protects the upper surface during etching.
Broadly, the present invention is embodied in a method for fabricating a suspended platform on a bonded-substrate. The bonded-substrate includes a platform substrate that is bonded to a base substrate. The method includes forming a dielectric layer on a support surface of the base substrate and then patterning and etching an interface surface of the dielectric layer to form a well that extends inward of the interface surface. The well has a preselected depth that leaves a thin layer of the dielectric layer covering the support surface. The base and platform substrates are then bonded to each other to form the bonded-substrate by urging the interface surface into contact with a mounting surface of the platform substrate and annealing the base and platform substrates to fusion bond the interface surface with the mounting surface. Once bonded, the well and the mounting surface form a sealed cavity. The platform substrate is thinned to form a thin membrane (of platform substrate material) over the sealed cavity. The membrane is patterned to define a platform feature and a flexure feature. The membrane is then etched to form trenches that extend all the way trough the membrane to the sealed cavity. The trenches define a platform (also called a microstructure) and one or more flexures that connect the platform with the platform substrate. Finally, the dielectric material beneath the platform is removed by applying a selective etch material. The selective etch material removes substantially all of the dielectric material from beneath the platform without substantially undercutting an interface between the interface layer and the mounting surface so that the chemical bond between the base substrate and the platform substrate is not damaged by the selective etch material.
One advantage of the present invention is that the thin layer of the dielectric material protects the support surface from pitting resulting from the etching of the membrane to form the trenches that define the platform and the flexures. Therefore, the problems associated with the pitting of the prior methods for fabricating a suspended structure are solved by the present invention.
Additionally, the present invention eliminates the need to perforate the platform in order to provide a path for the etch material to get to the dielectric layer beneath the platform. The trenches provide the necessary path for the etch material to get to the dielectric layer. Consequently, the platform of the present invention is devoid of holes, perforations, or the like that reduce the surface area of the platform.
In one embodiment of the present invention, prior to forming the well, the dielectric layer is planarized to form a substantially planar interface surface.
In another embodiment of the present invention, the dielectric layer is silicon oxide.
In yet another embodiment of the present invention, a thickness of the base substrate is reduced by backthinning a back surface of the base substrate.
In one embodiment of the present invention, the selective etch material is hydrofluoric acid.